1. Field of the Invention
The present invention relates methods of installing offshore oil and gas wells and protective structures and jackets therefor. Even more particularly the present invention relates to a method of installing large structural protector for offshore wells which uses an upstanding conductor pipe installed by a drilling vessel and wherein a lean-to truss having a pair of angled legs is initially supported and installed by the drilling vessel and subsequently forms a structural connection with the conductor pipe to define a three legged structure which includes the conductor pipe for protecting the well. The legs of the angled truss have pre-installed internal piling so that once placed by the rig, they can be quickly driven to affix the truss to the seabed.
A drilling vessel, for example, can be used to drill an oil or gas well in an offshore environment. Such a vessel can be a barge-like floating drilling rig, a jack-up rig, (which elevates a barge body on three or more legs), and in some cases a semi-submersible drilling rig. The vessel supports the drilling portion of the rig at a particular location during drilling of the well.
When the drilling of an offshore oil and gas well is completed, the vessel moves to another location leaving a well behind which includes an elongated generally vertical section of pipe known as the conductor pipe of for example two to three feet in diameter.
A supportive platform must be installed to protect the well from damage such as may occur from collisions by vessels or by weather. The structure is also used to provide a work area that can be occupied by personnel doing work associated with production of the oil and gas well.
Because of the offshore environment, it is often difficult, expensive and/or impossible to fabricate a platform on location. Rather, the practice has been to construct a jacket, support, or other heavy structure on land, and then transport that structure to the offshore environment where it is set in position at the well site. Between the time that the well is drilled and the conductor pipe placed in position, and the time that a more permanent production platform can be installed, the well must be protected. There is thus a need for a support which is quick and easy to assemble, and desirably inexpensive to manufacture. Such a support must necessarily be quickly erected on site in order to protect the well from storms which may occur in the days and weeks after drilling is completed and after the well drilling vessel, jack-up rig, or the like has left the drilling site. The structure can be removed and used at a second location if the first well is plugged and abandoned. Thus the structure has a dual function i.e., a permanent support, or if desired, removable.
2. General Background
Various platforms and offshore structures have been patented which have attempted to solve the problem of protecting the well from damage due to wind, weather, sea conditions, and from collision by other vessels. Many of these devices use a very permanent offshore jacket which is supported by the underlying sea bed. Most of these jackets include multiple legs which are generally vertical (sometimes slanted or battered) and which are interconnected by horizontal and diagonal cross members or chords. Some of these structures form connections with the well drive pipe or conductor pipe. For example, an article in the Jan. 19, 1987 issue of the Oil and Gas Journal describes a two-leg jacket that is clamped and welded to the well drive pipe to form a tripod. A deck support frame and deck module are supported from the two legs and braced to the drive pipe creating a rigid deck.
The Upson U.S. Pat. No. 2,927,435; the Horvath U.S. Pat. No. 4,027,734; the Evans U.S. Pat. No. 4,083,193; and the Liautaud U.S. Pat. No. 4,118,942 all disclose marine platforms which show permanent rigid jackets that include generally vertical columns interconnected by diagonal and/or horizontal cross members.
In the Upson Patent, a method is provided for constructing an offshore platform foundation which includes individually driving substantially vertically a plurality of piles in spaced apart relationship with the lower portions of the piles being driven to the sea bottom and the upper portions thereof extending to a substantial height above the water level and thereafter lowering to a substantial distance below the desired platform level. The frame has portions for securing the piles thereto, and also means for guiding and bracing the mid-portions of batter piles to be subsequently driven at angles to the vertically driven piles. As part of the method, the vertically driven piles are secured to the frame in spaced positions and a platform is secured to the upper portions of vertically driven piles at a desired level above the water and a substantial distance above the frame. The platform has batter pile guides and the batter piles are driven from the platform downwardly into the sea bottom at angles to the vertically driven piles while the batter piles are guided respectively by substantially spaced apart guides on the platform and guides on the frame.
In the Horvath U.S. Pat. No. 4,027,734, a method and technique for a non-vertical implacement of a supported conductor casing is provided. The method provides for the installation of a relatively short offset portion of the conductor casing at only the bottom of the conductor pipe string thereby utilizing this offset portion to force the conductor string in a desired direction and orientation by driving or forcing the conductor casing through a substratum in such manner that the casing will assume the desired curvature and direction as it advances through the substratum. In addition to the offset or dogleg segment at the bottom, keyway type guidebars can be added into the longitudinal direction to resist undesired rotation of conductor pipe. The platform area is shown as comprising a plurality of upstanding legs which are downwardly extending into the sea bed and a plurality of vertically spaced cross braces extending between the legs. Guides on the platform support the conductor.
The Evans U.S. Pat. No. 4,083,193 provides an offshore apparatus and a method for installing the offshore apparatus. The structure is mounted on the ocean floor and terminates in a platform located beneath the surface of the water to carry loads extending above the surface of the water. A plurality of legs extend generally vertically through a template base resting on the ocean floor and are anchored in the ocean bed below the base. A load platform is connected to the upper portions of the legs and is buoyant enough to maintain the legs in tension and to support the load which it carries. A stabilizer member is connected to the legs intermediate their length and is also buoyant to apply additional tension to the legs, and a series of mooring members are connected to the stabilizer member to minimize the side sway in response to ocean current. In the preferred method of installation, all three components are buoyant and are towed to the site of operation as a unit, and lowered to the ocean floor by ballasting them with sea water. The legs are installed through passages in the compartments and the platform and stabilizer member are de-ballasted to rise to their final position while the base remains ballasted and serves to maintain the integrity of the anchoring of the legs.
In the Liautaud U.S. Pat. No. 4,118,942 entitled "Marine Platform For Offshore Drilling Operations And The Like". There is provided a marine platform which comprises a caisson unit adapted to be placed onto the submarine ground at a desired offshore site. A tower like tubular structure comprising a plurality of columns extends vertically upwardly from the caisson unit with their lower ends integrally fixed to set caisson units. A deck unit adapted to be provided with petroleum drilling equipment or the like is integrally fixed to the upper ends of the columns. A floating unit is adapted to be displaceably and removably fixed to the columns.
A number of patents relate to devices which include a tripod-like tower structure which supports a centrally positioned conductor pipe. U.S. Pat. No. 4,553,878 issued to Willemse, U.S. Pat. No. 4,607,983 issued to Meek et al., U.S. Pat. No. 4,679,964 issued to Blandford and U.S. Pat. No. 4,687,380 issued to Meek et al., all disclose tripod like structures for supporting a central conductor like element in an offshore environment.
In the Willemes et al., U.S. Pat. No. 4,553,878, an offshore tower structure includes a base structure for positioning on and fixing to the sea bed a central column for containing services such as conductors, risers and extending in use from the base structure to above the water level for supporting a platform, at three supporting legs each extending between an upper portion of the column and the base structure at points spaced from the column provides support for the column and a bracing structure comprises a frame work lying intermediate the ends of the column in a plane perpendicular to the longitudinal axis of the column, the frame work connecting each pair of adjacent legs and each leg directly or indirectly with the column, and bracing elements between the column in the legs or between adjacent legs extend from the plane of the frame work to respected points at or adjacent the base of the structure.
The Meek et al. U.S. Pat. No. 4,607,983 provides a method of constructing an offshore tower structure having a base structure for positioning on the sea bed, a central enclosed tubular column containing services such as conductors and risers and extending from the base structure to above the water level for supporting a service platform which includes positioning the base structure on the sea bed, floating the column and legs over the base structure, lowering the column and legs under the base structure and securing them onto the latter.
The Blandford U.S. Pat. No. 4,679,964 provides a structure for use with a subsea well incorporating an external conductor pipe extending upwardly above the sea bed, a well support mini platform being a portion of the apparatus. A longitudinally split, flange-equipped, bolt joined elongate conductor clamp supported on a frame at the bottom thereof is provided having a plurality of appended upstanding braces. The support frame is adapted to be rested on a sea bed and held in place by a number of anchors driven into the sea bed at corner. Support frame is selectively installed after completion of a well wherein the conductor pipe extends above the sea bed. The support frame may subsequently be removed after installation of a permanent platform. In alternate embodiments, the support frame may be installed as a permanent structure. A boat landing and mini platform may be mounted on the conductor pipe or the conductor clamps supported by a support frame.
U.S. Pat. No. 4,687,380 issued to Meek provides a tower structure having a central column, three support legs and a base structure which can be anchored to the sea bed by means of piles. The legs and base structure are constructed as an integral pre-formed base unit with a sleeve attached to the upper ends of the legs.
The column is slidingly engaged within the sleeve of the base unit. A rigid connection is made between the column and the sleeve. In the erected structure, the primary load of the column is transmitted through the column and sleeve connection via the base unit to the sea bed.